使用 Matplotlib 绘制分段函数

Question

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

The above error is what I got for the below code intended to plot the piecewise function created. Can't figure it out.

import numpy as np
import matplotlib.pyplot as plt
def f(x):
    if x>=0 and x<=1:
        return x
    elif x>1 and x<=2:
        return 2-x
    else:
        return 0

xlist = np.linspace(0,1,num = 1000)
ylist = f(xlist)

plt.plot(ylist,xlist)
plt.show()

Answer 1

As per comments, you need to vectorize your method f (and also fix some mistakes):

import numpy as np
import matplotlib.pyplot as plt

def f(x):
    y = np.empty_like(x)
    mask1 = (0 <= x) & (x <= 1)
    mask2 = (1 < x) & (x <= 2)
    mask3 = np.logical_not((0 <= x) & (x <= 2)) #or ~((0 <= x) & (x <= 2))
    y[mask1] = x[mask1]
    y[mask2] = 2 - x[mask2]
    y[mask3] = 0
    return y

xlist = np.linspace(-1, 3, num = 1000)
ylist = f(xlist)

plt.plot(xlist, ylist)
plt.show()

You might also find that mask3 is not necessarry and vectorize a method f like this:

def f(x):
    y = np.full_like(x, fill_value=0) #or np.zeros_like(x)
    mask1 = (0 <= x) & (x <= 1)
    mask2 = (1 < x) & (x <= 2)
    y[mask1] = x[mask1]
    y[mask2] = 2 - x[mask2]
    return y

Answer 2

What I understood is, you are basically comparing an array with a number which should not be the case here, the code provided might help you with solution,

import numpy as np
import matplotlib.pyplot as plt
def f(x):
    if x>=0 and x<=1:
        return x
    elif x>1 and x<=2:
        return 2-x
    else:
        return 0

xlist = np.linspace(0,1,num = 1000,endpoint=False)
ylist = np.array([f(num) for num in xlist])

plt.plot(xlist,ylist)
plt.show()

Answer 3

You can use the function you have already defined and vectorize is using np.vectorize .

import numpy as np
import matplotlib.pyplot as plt
    def f(x):
    if x>=0 and x<=1:
        return x
    elif x>1 and x<=2:
        return 2-x
    else:
        return 0.    # make this change to ensure float values

xlist = np.linspace(0,1,num = 1000)
ylist = np.vectorize(f)(xlist)

plt.plot(ylist,xlist)
plt.show()

The issue you are running into in your code is that you are passing an array to f rather than applying f element-wise, so any comparison gives an array of True or False values; this is what is ambiguous in the error message. Using np.vectorize will change the function defined to apply to each element of the array and give you the desired output.